以太坊开发者文档04 智能合约剖析

智能合约剖析

一个智能合约是运行在以太坊上一个地址处的程序。它们由数据和在收到交易时执行的函数组成。以下是构成智能合约的概述。

先决条件

确保您首先阅读了有关智能合约的内容。本文档假设您已经熟悉 JavaScript 或 Python 等编程语言。

数据

任何合约数据都必须分配到一个位置：storage 或 memory。修改智能合约中的存储成本很高，因此您需要考虑数据应该存放在哪里。

Storage

持久性数据称为 storage，由状态变量表示。这些值永久存储在区块链上。你需要声明类型，以便合约可以跟踪它在编译时需要多少区块链上的存储空间。

// Solidity example
contract SimpleStorage {
    uint storedData; // State variable
    // ...
}

如果您已经编写过面向对象的语言，那么您可能会熟悉大多数类型。但是，如果您不熟悉以太坊开发，address 对您来说应该是新的。

address 类型可以保存相当于 20 字节或 160 位长的以太坊地址。它以带有前导 0x 的十六进制表示法返回。

其他的类型包括：

• boolean (布尔)
• integer (整型)
• fixed point numbers (定点数)
• fixed-size byte arrays (固定大小字节数组)
• dynamically-sized byte arrays (动态大小字节数组)
• Rational and integer literals (有理字面量和整型字面量)
• String literals (字符串字面量)
• Hexadecimal literals (十六进制字面量)
• Enums (枚举)

有关更多解释，请查看文档：

• See Vyper types
• See Solidity types

Memory

仅在合约函数执行的生命周期内存储的值称为内存变量。由于这些不会永久存储在区块链上，因此使用起来要便宜得多。

在 Solidity 文档中了解有关 EVM 如何存储数据（Storage、Memory 和 Stack）的更多信息。

环境变量

除了你在合约中定义的变量外，还有一些特殊的全局变量。它们主要用于提供有关区块链或当前交易的信息。

例如：

Prop	State variable	Description
block.timestamp	uint256	Current block epoch timestamp
msg.sender	address	Sender of the message (current call)

函数

用最简单的术语来说，函数可以获取信息或设置信息以响应传入的交易。

有两种类型的函数调用：

• internal - 不创建 EVM 调用

  • internal 函数和状态变量只能在内部访问（即从当前合约或从它派生的合约）

• external - 创建 EVM 调用

  • external 函数是合约接口的一部分，这意味着它们可以被其他合约和通过交易调用。外部函数 f 不能在内部调用（即 f() 不起作用，但 this.f() 起作用）

它们也可以是 public 或 private

• public 函数可以在合约内部调用，也可以通过消息在外部调用
• private 函数仅在定义它们的合约中可见，在派生合约中不可见

函数和状态变量都可以设为公有或私有。

在合约上使用函数更新状态变量：

// Solidity example
function update_name(string value) public {
    dapp_name = value;
}

• 将 string 类型的参数 value 传入函数：update_name
• 它被声明为 public，这意味着任何人都可以访问它
• 它没有被声明为 view，所以它可以修改合约状态

View functions

这些函数承诺不会修改合约数据的状态。常见的例子是“getter”函数——例如，您可以使用它来接收用户的余额。

// Solidity example
function balanceOf(address _owner) public view returns (uint256 _balance) {
    return ownerPizzaCount[_owner];
}

什么被认为是修改状态：

1. 写入状态变量
2. 触发事件
3. 创建其他合同
4. 使用 selfdestruct
5. 通过调用发送以太币
6. 调用任何未标记为 view 或 pure 的函数
7. 使用低级调用
8. 使用包含某些操作码的内联汇编

Constructor functions

构造(constructor)函数仅在首次部署合约时执行一次。与许多基于类的编程语言中的构造函数一样，这些函数通常将状态变量初始化为其指定值。

// Solidity example
// Initializes the contract's data, setting the `owner`
// to the address of the contract creator.
constructor() public {
    // All smart contracts rely on external transactions to trigger its functions.
    // `msg` is a global variable that includes relevant data on the given transaction,
    // such as the address of the sender and the ETH value included in the transaction.
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/units-and-global-variables.html#block-and-transaction-properties
    owner = msg.sender;
}

Built-in functions

除了您在合约中定义的变量和函数外，还有一些特殊的内置函数。最明显的例子是：

• address.send() - Solidity
• send(address) - Vyper

这些函数允许合约发送 ETH 给其他账户。

编写函数

你的功能需要：

• 参数变量和类型（如果它接受参数）
• internal/external 声明
• pure/view/payable 声明
• 返回类型（如果它返回一个值）

pragma solidity >=0.4.0 <=0.6.0;

contract ExampleDapp {
    string dapp_name; // state variable

    // Called when the contract is deployed and initializes the value
    constructor() public {
        dapp_name = "My Example dapp";
    }

    // Get Function
    function read_name() public view returns(string) {
        return dapp_name;
    }

    // Set Function
    function update_name(string value) public {
        dapp_name = value;
    }
}

一份完整的合约可能看起来像这样。这里构造函数为 dapp_name 变量提供了一个初始值。

事件和日志

事件让您可以从前端或其他订阅应用程序与您的智能合约进行通信。当交易被挖掘(minded)时，智能合约可以发出事件并将日志写入前端可以处理的区块链。

带注释的示例

这些是用 Solidity 编写的一些示例。如果您想玩代码，可以在 Remix 中与它们进行交互。

Hello world

// Specifies the version of Solidity, using semantic versioning.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/layout-of-source-files.html#pragma
pragma solidity ^0.5.10;

// Defines a contract named `HelloWorld`.
// A contract is a collection of functions and data (its state).
// Once deployed, a contract resides at a specific address on the Ethereum blockchain.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/structure-of-a-contract.html
contract HelloWorld {

    // Declares a state variable `message` of type `string`.
    // State variables are variables whose values are permanently stored in contract storage.
    // The keyword `public` makes variables accessible from outside a contract
    // and creates a function that other contracts or clients can call to access the value.
    string public message;

    // Similar to many class-based object-oriented languages, a constructor is
    // a special function that is only executed upon contract creation.
    // Constructors are used to initialize the contract's data.
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#constructors
    constructor(string memory initMessage) public {
        // Accepts a string argument `initMessage` and sets the value
        // into the contract's `message` storage variable).
        message = initMessage;
    }

    // A public function that accepts a string argument
    // and updates the `message` storage variable.
    function update(string memory newMessage) public {
        message = newMessage;
    }
}

代币

pragma solidity ^0.5.10;

contract Token {
    // An `address` is comparable to an email address - it's used to identify an account on Ethereum.
    // Addresses can represent a smart contract or an external (user) accounts.
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#address
    address public owner;

    // A `mapping` is essentially a hash table data structure.
    // This `mapping` assigns an unsigned integer (the token balance) to an address (the token holder).
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#mapping-types
    mapping (address => uint) public balances;

    // Events allow for logging of activity on the blockchain.
    // Ethereum clients can listen for events in order to react to contract state changes.
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#events
    event Transfer(address from, address to, uint amount);

    // Initializes the contract's data, setting the `owner`
    // to the address of the contract creator.
    constructor() public {
        // All smart contracts rely on external transactions to trigger its functions.
        // `msg` is a global variable that includes relevant data on the given transaction,
        // such as the address of the sender and the ETH value included in the transaction.
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/units-and-global-variables.html#block-and-transaction-properties
        owner = msg.sender;
    }

    // Creates an amount of new tokens and sends them to an address.
    function mint(address receiver, uint amount) public {
        // `require` is a control structure used to enforce certain conditions.
        // If a `require` statement evaluates to `false`, an exception is triggered,
        // which reverts all changes made to the state during the current call.
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/control-structures.html#error-handling-assert-require-revert-and-exceptions

        // Only the contract owner can call this function
        require(msg.sender == owner, "You are not the owner.");

        // Enforces a maximum amount of tokens
        require(amount < 1e60, "Maximum issuance exceeded");

        // Increases the balance of `receiver` by `amount`
        balances[receiver] += amount;
    }

    // Sends an amount of existing tokens from any caller to an address.
    function transfer(address receiver, uint amount) public {
        // The sender must have enough tokens to send
        require(amount <= balances[msg.sender], "Insufficient balance.");

        // Adjusts token balances of the two addresses
        balances[msg.sender] -= amount;
        balances[receiver] += amount;

        // Emits the event defined earlier
        emit Transfer(msg.sender, receiver, amount);
    }
}

数字资产

pragma solidity ^0.5.10;

// Imports symbols from other files into the current contract.
// In this case, a series of helper contracts from OpenZeppelin.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/layout-of-source-files.html#importing-other-source-files

import "../node_modules/@openzeppelin/contracts/token/ERC721/IERC721.sol";
import "../node_modules/@openzeppelin/contracts/token/ERC721/IERC721Receiver.sol";
import "../node_modules/@openzeppelin/contracts/introspection/ERC165.sol";
import "../node_modules/@openzeppelin/contracts/math/SafeMath.sol";

// The `is` keyword is used to inherit functions and keywords from external contracts.
// In this case, `CryptoPizza` inherits from the `IERC721` and `ERC165` contracts.
// Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#inheritance
contract CryptoPizza is IERC721, ERC165 {
    // Uses OpenZeppelin's SafeMath library to perform arithmetic operations safely.
    // Learn more: https://docs.openzeppelin.com/contracts/2.x/api/math#SafeMath
    using SafeMath for uint256;

    // Constant state variables in Solidity are similar to other languages
    // but you must assign from an expression which is constant at compile time.
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#constant-state-variables
    uint256 constant dnaDigits = 10;
    uint256 constant dnaModulus = 10 ** dnaDigits;
    bytes4 private constant _ERC721_RECEIVED = 0x150b7a02;

    // Struct types let you define your own type
    // Learn more: https://solidity.readthedocs.io/en/v0.5.10/types.html#structs
    struct Pizza {
        string name;
        uint256 dna;
    }

    // Creates an empty array of Pizza structs
    Pizza[] public pizzas;

    // Mapping from pizza ID to its owner's address
    mapping(uint256 => address) public pizzaToOwner;

    // Mapping from owner's address to number of owned token
    mapping(address => uint256) public ownerPizzaCount;

    // Mapping from token ID to approved address
    mapping(uint256 => address) pizzaApprovals;

    // You can nest mappings, this example maps owner to operator approvals
    mapping(address => mapping(address => bool)) private operatorApprovals;

    // Internal function to create a random Pizza from string (name) and DNA
    function _createPizza(string memory _name, uint256 _dna)
        // The `internal` keyword means this function is only visible
        // within this contract and contracts that derive this contract
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#visibility-and-getters
        internal
        // `isUnique` is a function modifier that checks if the pizza already exists
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/structure-of-a-contract.html#function-modifiers
        isUnique(_name, _dna)
    {
        // Adds Pizza to array of Pizzas and get id
        uint256 id = SafeMath.sub(pizzas.push(Pizza(_name, _dna)), 1);

        // Checks that Pizza owner is the same as current user
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/control-structures.html#error-handling-assert-require-revert-and-exceptions

        // note that address(0) is the zero address,
        // indicating that pizza[id] is not yet allocated to a particular user.

        assert(pizzaToOwner[id] == address(0));

        // Maps the Pizza to the owner
        pizzaToOwner[id] = msg.sender;
        ownerPizzaCount[msg.sender] = SafeMath.add(
            ownerPizzaCount[msg.sender],
            1
        );
    }

    // Creates a random Pizza from string (name)
    function createRandomPizza(string memory _name) public {
        uint256 randDna = generateRandomDna(_name, msg.sender);
        _createPizza(_name, randDna);
    }

    // Generates random DNA from string (name) and address of the owner (creator)
    function generateRandomDna(string memory _str, address _owner)
        public
        // Functions marked as `pure` promise not to read from or modify the state
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#pure-functions
        pure
        returns (uint256)
    {
        // Generates random uint from string (name) + address (owner)
        uint256 rand = uint256(keccak256(abi.encodePacked(_str))) +
            uint256(_owner);
        rand = rand % dnaModulus;
        return rand;
    }

    // Returns array of Pizzas found by owner
    function getPizzasByOwner(address _owner)
        public
        // Functions marked as `view` promise not to modify state
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/contracts.html#view-functions
        view
        returns (uint256[] memory)
    {
        // Uses the `memory` storage location to store values only for the
        // lifecycle of this function call.
        // Learn more: https://solidity.readthedocs.io/en/v0.5.10/introduction-to-smart-contracts.html#storage-memory-and-the-stack
        uint256[] memory result = new uint256[](ownerPizzaCount[_owner]);
        uint256 counter = 0;
        for (uint256 i = 0; i < pizzas.length; i++) {
            if (pizzaToOwner[i] == _owner) {
                result[counter] = i;
                counter++;
            }
        }
        return result;
    }

    // Transfers Pizza and ownership to other address
    function transferFrom(address _from, address _to, uint256 _pizzaId) public {
        require(_from != address(0) && _to != address(0), "Invalid address.");
        require(_exists(_pizzaId), "Pizza does not exist.");
        require(_from != _to, "Cannot transfer to the same address.");
        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");

        ownerPizzaCount[_to] = SafeMath.add(ownerPizzaCount[_to], 1);
        ownerPizzaCount[_from] = SafeMath.sub(ownerPizzaCount[_from], 1);
        pizzaToOwner[_pizzaId] = _to;

        // Emits event defined in the imported IERC721 contract
        emit Transfer(_from, _to, _pizzaId);
        _clearApproval(_to, _pizzaId);
    }

    /**
     * Safely transfers the ownership of a given token ID to another address
     * If the target address is a contract, it must implement `onERC721Received`,
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`;
     * otherwise, the transfer is reverted.
    */
    function safeTransferFrom(address from, address to, uint256 pizzaId)
        public
    {
        // solium-disable-next-line arg-overflow
        this.safeTransferFrom(from, to, pizzaId, "");
    }

    /**
     * Safely transfers the ownership of a given token ID to another address
     * If the target address is a contract, it must implement `onERC721Received`,
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`;
     * otherwise, the transfer is reverted.
     */
    function safeTransferFrom(
        address from,
        address to,
        uint256 pizzaId,
        bytes memory _data
    ) public {
        this.transferFrom(from, to, pizzaId);
        require(_checkOnERC721Received(from, to, pizzaId, _data), "Must implement onERC721Received.");
    }

    /**
     * Internal function to invoke `onERC721Received` on a target address
     * The call is not executed if the target address is not a contract
     */
    function _checkOnERC721Received(
        address from,
        address to,
        uint256 pizzaId,
        bytes memory _data
    ) internal returns (bool) {
        if (!isContract(to)) {
            return true;
        }

        bytes4 retval = IERC721Receiver(to).onERC721Received(
            msg.sender,
            from,
            pizzaId,
            _data
        );
        return (retval == _ERC721_RECEIVED);
    }

    // Burns a Pizza - destroys Token completely
    // The `external` function modifier means this function is
    // part of the contract interface and other contracts can call it
    function burn(uint256 _pizzaId) external {
        require(msg.sender != address(0), "Invalid address.");
        require(_exists(_pizzaId), "Pizza does not exist.");
        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");

        ownerPizzaCount[msg.sender] = SafeMath.sub(
            ownerPizzaCount[msg.sender],
            1
        );
        pizzaToOwner[_pizzaId] = address(0);
    }

    // Returns count of Pizzas by address
    function balanceOf(address _owner) public view returns (uint256 _balance) {
        return ownerPizzaCount[_owner];
    }

    // Returns owner of the Pizza found by id
    function ownerOf(uint256 _pizzaId) public view returns (address _owner) {
        address owner = pizzaToOwner[_pizzaId];
        require(owner != address(0), "Invalid Pizza ID.");
        return owner;
    }

    // Approves other address to transfer ownership of Pizza
    function approve(address _to, uint256 _pizzaId) public {
        require(msg.sender == pizzaToOwner[_pizzaId], "Must be the Pizza owner.");
        pizzaApprovals[_pizzaId] = _to;
        emit Approval(msg.sender, _to, _pizzaId);
    }

    // Returns approved address for specific Pizza
    function getApproved(uint256 _pizzaId)
        public
        view
        returns (address operator)
    {
        require(_exists(_pizzaId), "Pizza does not exist.");
        return pizzaApprovals[_pizzaId];
    }

    /**
     * Private function to clear current approval of a given token ID
     * Reverts if the given address is not indeed the owner of the token
     */
    function _clearApproval(address owner, uint256 _pizzaId) private {
        require(pizzaToOwner[_pizzaId] == owner, "Must be pizza owner.");
        require(_exists(_pizzaId), "Pizza does not exist.");
        if (pizzaApprovals[_pizzaId] != address(0)) {
            pizzaApprovals[_pizzaId] = address(0);
        }
    }

    /*
     * Sets or unsets the approval of a given operator
     * An operator is allowed to transfer all tokens of the sender on their behalf
     */
    function setApprovalForAll(address to, bool approved) public {
        require(to != msg.sender, "Cannot approve own address");
        operatorApprovals[msg.sender][to] = approved;
        emit ApprovalForAll(msg.sender, to, approved);
    }

    // Tells whether an operator is approved by a given owner
    function isApprovedForAll(address owner, address operator)
        public
        view
        returns (bool)
    {
        return operatorApprovals[owner][operator];
    }

    // Takes ownership of Pizza - only for approved users
    function takeOwnership(uint256 _pizzaId) public {
        require(_isApprovedOrOwner(msg.sender, _pizzaId), "Address is not approved.");
        address owner = this.ownerOf(_pizzaId);
        this.transferFrom(owner, msg.sender, _pizzaId);
    }

    // Checks if Pizza exists
    function _exists(uint256 pizzaId) internal view returns (bool) {
        address owner = pizzaToOwner[pizzaId];
        return owner != address(0);
    }

    // Checks if address is owner or is approved to transfer Pizza
    function _isApprovedOrOwner(address spender, uint256 pizzaId)
        internal
        view
        returns (bool)
    {
        address owner = pizzaToOwner[pizzaId];
        // Disable solium check because of
        // https://github.com/duaraghav8/Solium/issues/175
        // solium-disable-next-line operator-whitespace
        return (spender == owner ||
            this.getApproved(pizzaId) == spender ||
            this.isApprovedForAll(owner, spender));
    }

    // Check if Pizza is unique and doesn't exist yet
    modifier isUnique(string memory _name, uint256 _dna) {
        bool result = true;
        for (uint256 i = 0; i < pizzas.length; i++) {
            if (
                keccak256(abi.encodePacked(pizzas[i].name)) ==
                keccak256(abi.encodePacked(_name)) &&
                pizzas[i].dna == _dna
            ) {
                result = false;
            }
        }
        require(result, "Pizza with such name already exists.");
        _;
    }

    // Returns whether the target address is a contract
    function isContract(address account) internal view returns (bool) {
        uint256 size;
        // Currently there is no better way to check if there is a contract in an address
        // than to check the size of the code at that address.
        // See https://ethereum.stackexchange.com/a/14016/36603
        // for more details about how this works.
        // TODO Check this again before the Serenity release, because all addresses will be
        // contracts then.
        // solium-disable-next-line security/no-inline-assembly
        assembly {
            size := extcodesize(account)
        }
        return size > 0;
    }
}

延伸阅读

查看 Solidity 和 Vyper 的文档以获得更完整的智能合约概述：

• Solidity
• Vyper